The present invention relates to a dry process developing apparatus for use with an apparatus for electrophotography.
An electrophotographic apparatus generally includes a developing apparatus for processing into a toner image a latent image which is electrostatically formed on a photoconductive drum by image light reflected from an original document. Among various apparatuses of the kind concerned, a dry process developing apparatus comprises a developing roller located to face the photoconductive drum between a charger and a transfer charger, a bladed toner supply roller positioned at the opposite side to the drum with respect to the developing roller in order to supply the toner to the developing roller, and a casing having the toner supply roller and developing roller thereinside and formed with an opening which faces the drum. Inside the casing, the toner is supplied from the toner supply roller onto the developing roller to form a magnet brush on the developing roller due to the attraction by a plurality of magnets installed in the developing roller. The magnet brush is brought into contact with the drum through the opening in accordance with the rotation of the developing roller.
A problem has existed in this type of developing apparatus in that the fine toner particles tend to fly apart through the gap between the drum and the upper end of the opening of the casing as well as through the gap between the drum and the lower casing end. The leaked toner particles smear an imaging system, transfer unit and other arrangements located around the developing device, thereby degrading the image quality to a critical degree unless frequent services are effected. Particularly, the casing lower end and drum are usually spaced a certain distance large enough to prevent a toner image carried on the drum from being damaged, allowing a substantial amount of toner particles to leak through the spacing. This part of the toner smears a lamp for image transfer which extends along the axis of the developing roller, resulting in a change in the intensity of illumination by the lamp. Any inadequate intensity level of illumination would effect the quality of reproduced images. Additionally, the toner dropping along the drum contaminates the transfer charger or a sheet guide and, as a result, renders the effect of the transfer charger uneven in the lengthwise direction while smearing sheets fed along the sheet guide.
The toner leakage described above becomes particularly prominent in the case of a two-component magnetic toner. When the toner is uncoupled from the carrier while being routed through various paths during development, fine particles thereof are made afloat.
Meanwhile, the developing apparatus of this type has a toner supply device which is located in an upper portion thereof. It is desirable that the toner fed downwardly from the toner supply device be caused to drop directly on the toner supply roller. However, the current tendency is to space the toner supply device as much as possible from the drum in the horizontal direction due to various layout limitations. For example, because the toner supply device which includes a toner container occupies a substantial space, it has to be located to avoid its interference with the imaging system.
Therefore, the toner from the container drops not on the toner supply roller but on the casing adjacent to the toner supply roller. As the time elapses, the toner dropping on the casing forms a heap. It is not until the heap of toner slides down that the toner is scooped into the gap between the blades of the toner supply roller and thereby fed to the developing roller. A certain period of time, therefore, expires before the toner is actually supplied by the toner supply roller to the developing roller. Should the development occur within this "transitional" period of time, the resulting image would be poor quality due to the short supply of toner.